Computed tomography (CT) is a standard imaging technique for radiology diagnosis. However, the use of an X-ray tube comprising only a single focal spot sometimes causes reconstruction problems in particular when large objects have to be examined. Thereby, for a certain viewing angle border regions of the object may not be located within the X-ray beam originating from the single focal spot and impinging onto the detector. This has the effect that for these border regions only a reduced number of projection views are available such that the quality of the three-dimensional (3D) reconstruction of the object under examination is reduced. In particular reconstruction artifacts may be generated, which erroneously indicate structures, which are in reality not existent.
In order to increase the available number of projection views also for border regions, dual focal spot X-ray tubes can be used. Thereby, for each viewing angle of a CT scanning unit comprising the X-ray source and the X-ray detector two two-dimensional (2D) X-ray attenuation datasets representing two different projection angles can be generated. Of course, the spatial distance between the two focal spots defines the angular difference between these two 2D X-ray attenuation datasets.
US 2005/0063514 A1 discloses an extended multi-spot CT X-ray source. The described X-ray source comprises an electron gun capable of producing a plurality of electron beams, each electron beam being focused at a predetermined distance and aimed in a predetermined direction. The described X-ray source further comprises and a plurality of targets positioned to receive the electron beams and generate X-rays in response thereto, whereby each target comprises a predetermined focal spot thereon. Each electron beam is synchronized to strike, at an appropriate time, a predetermined target comprising a predetermined focal spot thereon.
US 2005/0053189 A1 discloses an X-ray CT apparatus capable of imaging a subject based on X-rays of multiple energy levels. The CT apparatus comprises an X-ray tube, which generates X-rays from multiple focal points of different 3D positions sequentially on a time-division basis. A plurality of filters are provided, which implement the filtering individually for the X-rays generated individually from the focal points. The anode of the X-ray tube has multiple impingement portions where electrons released by a cathode of the X-ray tube impinge at multiple positions on the trajectory of electrons sequentially on a time-division basis.
U.S. Pat. No. 6,282,263 B1 discloses an X-ray generator comprising an evacuated and sealed X-ray tube, an electron gun, an X-ray target, an internal electron mask, and an X-ray window consisting of a thin tube of material with low X-ray absorption and high mechanical strength. The generator preferably includes a system for focusing and steering the electron beam onto the X-ray target, a cooling system to cool the target material, kinematic mounts to allow precise and repeatable mounting of X-ray devices for focusing the X-ray beam, and X-ray focusing devices of varying configurations and methods.
U.S. Pat. No. 6,778,633 B1 discloses an X-ray generator comprising an evacuated and sealed X-ray tube. The X-ray tube contains an electron gun and an X-ray target. An electron beam is produced by the electron gun consisting of a filament just inside the aperture of a Wehnelt grid. The Wehnelt grid is biased negatively with respect to the filament. Two sets of beam deflection coils are, employed in two planes, mounted between the anode of the electron gun and a focusing lens to center the beam. Between the focusing lens and the target is an air-cored quadrupole magnet, which acts as a stigmator in that it turns the circular cross-section of the beam into an elongated one.
U.S. Pat. No. 6,895,079 B2 discloses a multiple focal spot X-ray inspection system comprising an X-ray source having means for generating more than one beam defining an inspection plane, the beams being substantially parallel to each other. The X-ray inspection system further comprises an X-ray detector having a plurality of detector arrays, each of which is aligned with one of the beams. The means for generating more than one beam may include an electron gun and means for steering an electron beam generated by the gun to multiple focal spots on a target.
U.S. Pat. No. 6,252,935 B1 discloses an X-ray radiator having an x-ray tube with a deflection arrangement that deflects the electron beam of the x-ray tube dependent on a control signal such that the position of the focal spot on the anode corresponds to a reference position changing as a function of time. A detector acquires the actual position of the focal spot and a control unit is supplied with the actual value signal and a reference signal and generates the control signal. The described X-ray radiator has the disadvantage that a precise focusing of the electron beam gets rather difficult in particular when the spatial distance between various focal spot positions is comparatively large.
There may be a need for providing a multiple focal X-ray tube, which allows for an easy and reliable focusing of the different electron beams being assigned to different focal spot positions.